Passive alternator depressurization and cooling system

ABSTRACT

A pressure reduction system may include an alternator with a casing and a rotor positioned, at least in part, within a cavity defined by the casing. The pressure reduction system may also include a mass management system that includes a control tank configured to be maintained at a tank pressure lower than a cavity pressure within the cavity of the alternator, thereby forming a pressure differential. A first transfer conduit may transfer a working fluid from the cavity of the alternator to the control tank via the pressure differential. The mass management system may be positioned at an elevation above the alternator, and include a refrigeration loop configured to cool the working fluid contained within the control tank. A second transfer conduit may fluidly couple the alternator and the mass management system, and may transfer the cooled working fluid from the control tank to the cavity via gravitational force.

This application claims the benefit of U.S. Prov. Appl. No. 62/093,544,filed Dec. 18, 2015. This application is incorporated herein byreference in its entirety to the extent consistent with the presentapplication.

Waste heat is often created as a byproduct of industrial processes whereflowing streams of high-temperature liquids, gases, or fluids must beexhausted into the environment or removed in some way in an effort tomaintain the operating temperatures of the industrial process equipment.Some industrial processes use heat exchanger devices to capture andrecycle waste heat back into the process. However, the capturing andrecycling of waste heat is generally infeasible by industrial processesthat use high temperatures, have insufficient mass flow, or includeother unfavorable conditions.

Waste heat can be converted into useful energy by a variety of heatengine systems that employ thermodynamic methods, such as Rankinecycles. Rankine cycles and similar thermodynamic methods are typicallysteam-based processes that recover and use waste heat to generate steamfor driving a turbine or other type of expansion device. The turbine isthen connected to an electric generator, such as an alternator, which isused to supply electricity to an electrical bus or grid (e.g., analternating current bus) that usually has a varying load or demand overtime.

In certain circumstances, such as, for example, peak demand, alternatorsand associated components thereof (e.g., rotor, stator, and bearings)may be susceptible to overheating. To eliminate or reduce suchoverheating, methods employed have included cooling the alternator byusing a blower or a fan to circulate gas or fluid through an externalheat exchanger then through the alternator. However, using such coolingcomponents (e.g., blower/fan, heat exchanger, piping, and valves)generally incurs additional expenses, increases installation andmaintenance time, and creates a larger footprint.

Alternators may also be susceptible to over pressurization of thealternator cavity, which may occur when additional working fluid fromthe expansion device leaks past the bearings and seals encasing therotor of the alternator. Over pressurization of the alternator oftenresults in reduced efficiency, and in some instances, complete shutdownof the alternator.

What is needed, then, is a system for use in a heat engine system thatefficiently cools the alternator and efficiently reduces pressure withinthe alternator as needed.

In one embodiment, a pressure reduction system may include analternator. The alternator may include a casing and a rotor positioned,at least in part, within a cavity defined by the casing. The pressurereduction system may also include a mass management system having acontrol tank configured to be maintained at a tank pressure lower than acavity pressure within the cavity of the alternator, thereby forming apressure differential therebetween. A first transfer conduit may beconfigured to transfer a working fluid from the cavity of the alternatorto the control tank via the pressure differential.

In another embodiment, a cooling system may include an alternator havinga casing and a rotor positioned, at least in part, in a cavity definedby the casing. The cooling system may also include a mass managementsystem having a control tank configured to be positioned at an elevationabove the alternator. The control tank may include a refrigeration loopconfigured to cool a working fluid contained within the control tank.The cooling system may include a first transfer conduit fluidly couplingthe alternator and the mass management system, and the first transferconduit may be configured to transfer the working fluid from the cavityto the control tank. The cooling system may also include a secondtransfer conduit fluidly coupling the alternator and the mass managementsystem, and the second transfer conduit may be configured to transferthe cooled working fluid from the control tank to the cavity viagravitational force.

In another embodiment, a heat engine system may include an expansiondevice in a working fluid circuit, and the expansion device may beconfigured to receive a working fluid at an expansion device inlet at ahigh pressure. The expansion device may output the working fluid at alow pressure, and further convert a pressure drop in the working fluidto mechanical energy. The heat engine system may include an alternatorfluidly coupled to the expansion device. The alternator may convert themechanical energy to electrical energy, and include a casing and a rotorpositioned at least in part in a cavity defined within the casing. Thecavity of the alternator may further be configured to receive a portionof the working fluid from the expansion device. The heat engine systemmay include a mass management system that includes a control tankconfigured to be maintained at a tank pressure substantially lower thana cavity pressure within the cavity to form a pressure differentialtherebetween. A first transfer conduit may be configured to transfer theworking fluid from the cavity of the alternator to the control tank viathe pressure differential. The heat engine system may include a pumpfluidly coupled to the expansion device and configured to receive theworking fluid at a low pressure and output the working fluid at a highpressure. A recuperator may be fluidly coupled to the pump andconfigured to heat the working fluid exiting the pump. The heat enginesystem may further include a waste heat exchanger fluidly coupled to therecuperator. The waste heat exchanger may be configured to further heatthe working fluid after exiting the recuperator and before entering theexpansion device.

The present disclosure is best understood from the following detaileddescription when read with the accompanying Figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 depicts an exemplary heat engine system including a system fordepressurizing and cooling the alternator, according to one or moreembodiments disclosed herein.

FIG. 2 depicts an exemplary system for depressurizing and cooling analternator, according to one or more embodiments disclosed herein.

FIG. 3 is a graph depicting fluid friction loss and refrigeration workas a function of pressure in an alternator, according to one or moreembodiments disclosed herein.

It is to be understood that the following disclosure describes severalexemplary embodiments for implementing different features, structures,or functions of the invention. Exemplary embodiments of components,arrangements, and configurations are described below to simplify thepresent disclosure; however, these exemplary embodiments are providedmerely as examples and are not intended to limit the scope of theinvention. Additionally, the present disclosure may repeat referencenumerals and/or letters in the various exemplary embodiments and acrossthe Figures provided herein. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various exemplary embodiments and/or configurationsdiscussed in the various Figures. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact.Finally, the exemplary embodiments presented below may be combined inany combination of ways, i.e., any element from one exemplary embodimentmay be used in any other exemplary embodiment, without departing fromthe scope of the disclosure.

Additionally, certain terms are used throughout the followingdescription and claims to refer to particular components. As one skilledin the art will appreciate, various entities may refer to the samecomponent by different names, and as such, the naming convention for theelements described herein is not intended to limit the scope of theinvention, unless otherwise specifically defined herein. Further, thenaming convention used herein is not intended to distinguish betweencomponents that differ in name but not function. Additionally, in thefollowing discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to.” All numericalvalues in this disclosure may be exact or approximate values unlessotherwise specifically stated. Accordingly, various embodiments of thedisclosure may deviate from the numbers, values, and ranges disclosedherein without departing from the intended scope. It should also beappreciated that the term “about,” as used herein, in conjunction with anumeral refers to a value that is +/−5% (inclusive) of that numeral,+/−10% (inclusive) of that numeral, or +/−15% (inclusive) of thatnumeral. It should further be appreciated that when a numerical range isdisclosed herein, any numerical value falling within the range is alsospecifically disclosed. Furthermore, as it is used in the claims orspecification, the term “or” is intended to encompass both exclusive andinclusive cases, i.e., “A or B” is intended to be synonymous with “atleast one of A and B,” unless otherwise expressly specified herein.

Embodiments of the disclosure generally provide a system for coolingand/or reducing pressure in an alternator. One or more embodiments ofthe disclosure also provide a heat engine system including the systemfor cooling and/or reducing pressure in the alternator.

FIG. 1 depicts a heat engine system 10 that includes a system 100 forheating and cooling an alternator 105. The heat engine system 10 mayalso be referred to as a thermal engine system, an electrical generationsystem, a waste heat or other heat recovery system, and/or a thermal toelectrical energy system, as described in one of more embodimentsherein. The heat engine system 10 may include a waste heat system 12 anda power generation system 220 coupled to and in thermal communicationwith each other via a working fluid circuit 202. The working fluidcircuit 202 may contain the working fluid (e.g., sc-CO₂) and may have ahigh pressure side and a low pressure side, which will be describedherein. A heat source stream 11 may flow through heat exchangers 20 and30 disposed within the waste heat system 12. Each of the heat exchangers20 and 30, independently, may be fluidly coupled to and in thermalcommunication with the high pressure side of the working fluid circuit202, configured to be fluidly coupled to and in thermal communicationwith a heat source stream 11, and configured to transfer thermal energyfrom the heat source stream 11 to the working fluid within the highpressure side of the working fluid circuit 202. Thermal energy may beabsorbed by the working fluid within the working fluid circuit 202 andconverted to mechanical energy by flowing the heated working fluidthrough one or more expanders or turbines.

The heat engine system 10 may further include at least one pump, such asa turbopump 260, disposed within the working fluid circuit 202 andfluidly coupled between the low pressure side and the high pressure sideof the working fluid circuit 202. The turbopump 260 may be configured tocirculate and to pressurize the working fluid throughout the workingfluid circuit 202. The turbopump 260 may include a pump portion 262coupled with a turbine 264. The low pressure side of the working fluidcircuit 202 extends from an outlet of the turbine 264 to the inlet ofthe pump portion 262 of the turbopump 260. The high pressure side of theworking fluid circuit 202 extends from the inlet of the pump portion 262to the outlet of the turbine 264.

The turbine 264 of the turbopump 260 may be fluidly coupled to theworking fluid circuit 202 downstream of the heat exchanger 20 and thepump portion 262 of the turbopump 260 may be fluidly coupled to theworking fluid circuit 202 upstream of the heat exchanger 20. In oneembodiment, the turbine 264 may be downstream of multiple heatexchangers, such as heat exchanger 20 and 30, within the working fluidcircuit 202. In one example, the turbine 264 may be configured toreceive and be powered by the working fluid passing through andabsorbing thermal energy from the heat exchanger 20. In one example, theturbine 264 may be configured to receive and be powered by the workingfluid passing through and absorbing thermal energy from more than oneheat exchanger, such as heat exchangers 20 and 30. The turbopump 260 mayfurther include a driveshaft 267 coupled between the turbine 264 and thepump portion 262.

The turbine 264 may be fluidly coupled to and in thermal communicationwith the working fluid, and configured to convert thermal energy tomechanical energy by a pressure drop in the working fluid flowingbetween the high and the low pressure sides of the working fluid circuit202. An alternator 105 may be coupled to the turbine 264 and configuredto convert the mechanical energy into electrical energy. A power outletmay be electrically coupled to the alternator 105 and configured totransfer the electrical energy from the alternator 105 to an electricalgrid. The power generation system 220 may further include a driveshaft230 coupled between the turbine 264 and the alternator 105. In oneembodiment, the driveshaft 267 may be integral with the driveshaft 230,or may be a solitary driveshaft. The power generation system 220 mayfurther contain a bearing housing 238 which substantially encompasses orencloses the bearings disposed within the power generation system 220.

Exemplary structures of the bearing housing 238 may completely orsubstantially encompass or enclose the bearings as well as all or partof turbines, generators, pumps, driveshafts, or other components shownor not shown for the heat engine system 10. The bearing housing 238 maycompletely or partially include structures, chambers, cases, housings,such as turbine housings, generator housings, driveshaft housings,driveshafts that contain bearings, gearbox housings, derivativesthereof, or combinations thereof. FIG. 1 depicts the bearing housing 238containing all or a portion of the turbine 264, the alternator 105, thedriveshafts 230 and 267, and the pump portion 262 of the powergeneration system 220. In some examples, the housing of the turbine 264may be coupled to and/or forms a portion of the bearing housing 238.

FIG. 2 shows the alternator 105 in more detail, and also depicts thesystem 100 for depressurizing and cooling the alternator 105. Thealternator 105 may include a casing 110 defining a cavity 115 in which,at least in part, a rotor 125 is positioned and configured to spin athigh speed. The rotor 125 may be integral with the driveshafts 230 or267, or the rotor 125 may form a solitary driveshaft with thedriveshafts 230 and/or 267. In one embodiment, the rotor 125 may have arotational speed between about 20,000 RPM and about 40,000 RPM. Thecavity 115 may contain a working fluid, which in one embodiment, may beor include carbon dioxide. Further, in one embodiment, the working fluidmay be carbon dioxide and at least a portion of the working fluid may bein a supercritical state. However, other working fluids including, butnot limited to ammonia and a combination of working fluids, arecontemplated. The working fluid in the cavity 115 may be containedwithin the alternator 105 by a shaft seal 120 positioned between therotor 125 and the casing 110 at one end of the alternator 105. The shaftseal 120 may be a labyrinth seal, a double seal, a dynamically pressurebalanced seal, a dry gas seal, or any other sealing mechanism configuredto reduce leakage flow of the working fluid into or out of the casing110.

The system 100 for depressurizing and cooling the alternator 105 mayinclude a mass management system 150 configured to control the pressureand temperature within the cavity 115 of the alternator 105. Asdiscussed in more detail below, the mass management system 150 mayinclude a control tank 155 configured to receive and store working fluidfrom the alternator 105 and, in addition, to disperse working fluid tothe alternator 105. The control tank 155 may be maintained at arelatively low pressure, such as, for example, about 0.5 MPa to about 2MPa.

The mass management system 150 may include a closed refrigeration loop160 positioned, at least in part, within the control tank 155 in orderto maintain a low pressure of the working fluid within the control tank155. The refrigeration loop 160 may include a cool fluid source 161,which may be water, seawater, nitrogen, or any other fluid, that mayflow through a conduit into the control tank 155. The cooled fluid mayflow through a condenser 162 to further condense the cooled fluid. Aftercondensing the cooled fluid, the cooled fluid may flow through a heatexchanger 163, wherein heat is transferred from the working fluid to thecooled fluid, and wherein the fluid flows into a compressor 164 and outto the cooled fluid source. The closed refrigeration loop 160 maytherefore cool the working fluid contained within the control tank 155.The pressure of the control tank 155 may be lower than the pressurewithin the alternator 105, which may be, for example, around about 0.5MPa to about 11 MPa. In one embodiment, the control tank 155 may bepositioned at an elevation above the alternator 105.

The system 100 may include a first transfer conduit 130 fluidly couplingthe alternator 105 and the mass management system 150, thereby forming afirst fluid passageway therebetween. More specifically, the firsttransfer conduit 130 may fluidly couple the cavity 115 of the alternator105 and the control tank 155. In one embodiment, the first transferconduit 130 may provide the first fluid passageway between a lowerportion of the alternator 105 and an upper portion of the control tank155.

The first transfer conduit 130 may include a valve 135 positioned withinthe first fluid passageway between the alternator 105 and the controltank 155 and configured to control fluid flow therebetween. As such, thevalve 135 may prevent or throttle the flow of working fluid between thealternator 105 and the control tank 155. In one embodiment, the valve135 may be a check valve to prevent working fluid from flowing from thecontrol tank 155 to the alternator 105 via the first transfer conduit130. The system 100 may also include a heat exchanger 140 fluidlycoupled with the first transfer conduit 130 and configured to cool theworking fluid moving from the alternator 105 to the control tank 155prior to the working fluid entering the control tank 155 in order toreduce the cooling duty of the closed refrigeration loop 160.

As previously discussed, the control tank 155 may be maintained at alower pressure than the cavity 115 of the alternator 105. Therefore,working fluid may flow through the first transfer conduit 130, from thealternator 105 to the control tank 155, based on a positive pressuredifferential. Such flow may be passive, or in other words, without aidof a pump or other like equipment. In addition, the positive pressuredifferential may allow working fluid to be transferred from thealternator 105 to the control tank 155 to optimize the operation of thealternator 105. For example, the pressure within the cavity 115 mayincrease as working fluid from the turbine 264 leaks past the shaft seal120 into the cavity 115. As shown in FIG. 3, a higher fluid pressurewithin the cavity 115 results in greater power loss within thealternator 105. However, in embodiments of the system 100 describedherein, as working fluid leaks past the turbine 264 shaft seal 120 andinto the cavity 115, working fluid may flow from the alternator 105 tothe control tank 155 via the first transfer conduit 130 based on thepressure differential between the cavity 115 and the control tank 155 toprevent additional power loss. In one embodiment, the flow rate of theworking fluid from the cavity 115 to the control tank 155 may be about600 grams per second. The flow rate may be dependent on, amongst otherfactors, the leak rate of working fluid from the turbine 264 to thealternator 105.

The system 100 further includes a second transfer conduit 165 fluidlycoupling the mass management system 150 and the alternator 105. Morespecifically, the second transfer conduit 165 may fluidly couple thecontrol tank 155 and the cavity 115 of the alternator 105 to form asecond fluid passageway therebetween. In one embodiment, the secondtransfer conduit 165 may form the second fluid passageway between alower portion of the control tank 155 and an upper portion of thealternator 105. The second transfer conduit 165 may include a valve 170positioned within the second fluid passageway between the control tank155 and the alternator 105 and configured to control fluid flowtherebetween. As such, the valve 170 may prevent or throttle the flow ofworking fluid between the control tank 155 and the alternator 105. Inone embodiment, the valve 170 may be a check valve to prevent fluid fromflowing from the alternator 105 to the control tank 155 via the secondtransfer conduit 165.

As discussed, the control tank 155 may be positioned at an elevationabove the alternator 105, such that working fluid may be gravity fed viathe second transfer conduit 165 from the control tank 155 to the cavity115 of the alternator 105. In one embodiment, the working fluid may exitthe control tank 155 at a flow rate of about 500 grams per second. Inother embodiments, the flow rate of the working fluid exiting thecontrol tank 155 may be greater or lesser depending on, amongst otherfactors the windage within the cavity 115 and the elevation of thecontrol tank 155 above the alternator 105. Further, because the workingfluid within the control tank 155 may be cooled by the refrigerationloop 160, the working fluid flowing from the control tank 155 to thealternator 105 may cool the alternator 105, which may be heated by thefluid friction (windage) generated by the rotation of the rotor 125within the cavity 115 of the alternator 105. Because the control tank155 may be vertically positioned at an elevation above the alternator105, the cooling of the alternator 105 may be accomplished in a passivemanner.

The system 100 may also include a return conduit 175 fluidly coupledwith the second transfer conduit 165 at a location 185 between thecontrol tank 155 and the alternator 105. The return conduit 175 may beconfigured to transfer working fluid from the second transfer conduit165 to the heat engine system 10. A transfer pump 180 may be fluidlycoupled with the return conduit 175 and configured to maintain arelatively constant amount of mass in the heat engine system 10 bytransferring the working fluid from the mass management system 150 tothe heat engine system 10 at a flow rate relatively equal to the flowrate of the working fluid entering the alternator 105 through the shaftseal 120 from the heat engine system 10. In one embodiment, the flowrate of the working fluid through the return conduit 175 may be about100 grams per second. In other embodiments, the flow rate of the workingfluid through the return conduit 175 may be greater or lesser depending,amongst other factors, on the leak rate of working fluid from theturbine 264 to the alternator 105.

In operation, the rotor 125, which may be driven by the turbine 264, mayrotate at a high speed, e.g., about 20,000 RPM to about 40,000 RPM,within the cavity 115 at least partially filled with working fluid. Inoperation, as the rotor 125 rotates, working fluid may leak past theshaft seal 120 and into the cavity 115 from the turbine 264 of the heatengine system 10. The additional working fluid entering the cavity 115may result in an increase of pressure within the cavity 115. Further,the rotation of the rotor 125 may induce fluid friction which leads toheating, or windage. As shown in FIG. 3, power loss within thealternator 105 increases as fluid pressure increases within the cavity115 inducing windage. Accordingly, if the rotor 125 continues to rotatewithin the cavity 115 without intervention, the temperature within thealternator 105 will increase, which may lead to overheating. However, inthe system 100 provided herein, as working fluid leaks into the cavity115, working fluid may flow from the alternator 105 to the control tank155 via the first transfer conduit 130 based on the pressuredifferential between the cavity 115 and the control tank 155.

The working fluid in the control tank 155 may be cooled by the massmanagement system 150 via the refrigeration loop 160. Because thecontrol tank 155 may be positioned at an elevation above the alternator105, the cooled working fluid may be transferred to the alternator 105via the second transfer conduit 165 by gravitational force, therebycooling the alternator 105. In addition, as working fluid is added tothe system 100 from leakage through the shaft seal 120, working fluidmay be returned back to the heat engine system 10 via the return conduit175 and the transfer pump 180 as the working fluid flows out of thecontrol tank 155.

Turning back to the heat engine system 10 illustrated in FIG. 1, theheat engine system 10 may further include at least one recuperator 216fluidly coupled to the working fluid circuit and operative to transferthermal energy between the high and low pressure sides of the workingfluid circuit 202. In some examples, the recuperator 216 may beconfigured to transfer the thermal energy from the low pressure side tothe high pressure side. The heat engine system 10 may further include acooler 274 in thermal communication with the working fluid contained inthe low pressure side of the working fluid circuit 202 and configured toremove thermal energy from the working fluid in the low pressure side.In some examples, the cooler 274 may be a condenser configured tocontrol a temperature of the working fluid in the low pressure side ofthe working fluid circuit 202 by transferring thermal energy from theworking fluid in the low pressure side to a cooling loop outside of theworking fluid circuit 202.

In one embodiment, the cooler 274 may circulate a coolant from a coolingcircuit 200 to cool the working fluid contained in the low pressure sideof the working fluid circuit 202. In one embodiment, the coolantcirculating through the cooling circuit 200 may be water, such asfreshwater. A pump 210 may be disposed within the cooling circuit 200 tocirculate the coolant through the cooling circuit 200. A cooler 215 mayalso be disposed within the cooling circuit 200 to transfer thermalenergy from the coolant moving through the cooling circuit 200. In oneembodiment, the cooler 215 may circulate seawater to transfer thermalenergy from the coolant to the seawater. For example, seawater may enterthe cooler 215 via an inlet line 212, and seawater may exit the cooler215 via an outlet line 214.

The heat engine system 10 may also include another mass managementsystem (MMS) 270 fluidly coupled to the working fluid circuit 202. TheMMS 270 may include a mass control tank 286 fluidly coupled to the lowpressure side of the working fluid circuit 202 and configured toreceive, store, and deliver the working fluid. The mass control tank 286and the working fluid circuit 202 may share the working fluid (e.g.,carbon dioxide) such that the mass control tank 286 may receive, store,and disperse the working fluid during various operational steps of theheat engine system 10. In one embodiment, the mass control tank 286 mayreceive additional working fluid via a feed line inlet 288.

The MMS 270 may include an inventory return line 72 fluidly coupled toand between the mass control tank 286 and the low pressure side of theworking fluid circuit 202, such as downstream of the condenser 274. Asdepicted in FIG. 1, a fluid line 68 may be fluidly coupled with andextend from the outlet of the condenser 274, and the inventory returnline 72 may be fluidly coupled to and extend from the fluid line 68 tothe mass control tank 286. The MMS 270 may also include a pump 70fluidly coupled to the mass control tank 286 and configured to transferthe working fluid from the mass control tank 286 to the low pressureside of the working fluid circuit 202 by an inventory supply line 82.Accordingly, the MMS 270 may receive the working fluid from the workingfluid circuit 202, store the working fluid for subsequent use, anddeliver the working fluid into the working fluid circuit 202.

It is contemplated that the mass management system 270 for use with thepump portion 282 of the heat engine system 10 may be combined with themass management system 150 for use with the alternator 105, as describedabove. To that extent, the mass control tank 286 of the heat enginesystem 10 for use with the pump portion 282 and the control tank 155 ofthe mass management system 150 for use with the alternator 105 may becombined into a single tank. The single tank may control the additionand/or removal of working fluid to the high pressure side of the heatengine system 10, the low pressure side of the heat engine system 10,and/or the alternator cavity 115.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the present disclosure. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

1. A pressure reduction system comprising: an alternator comprising acasing and a rotor positioned, at least in part, within a cavity definedby the casing; a mass management system comprising a control tankconfigured to be maintained at a tank pressure lower than a cavitypressure within the cavity to form a pressure differential therebetween;and a first transfer conduit configured to transfer a working fluid fromthe cavity of the alternator to the control tank via the pressuredifferential.
 2. The pressure reduction system of claim 1, furthercomprising a second transfer conduit configured to transfer the workingfluid from the control tank to the cavity.
 3. The pressure reductionsystem of claim 1, wherein the control tank comprises a closedrefrigeration loop configured to cool the working fluid.
 4. The pressurereduction system of claim 1, further comprising a heat exchangerconfigured to cool the working fluid prior to the working fluid enteringthe control tank.
 5. The pressure reduction system of claim 2, furthercomprising: a first valve configured to control a flow of the workingfluid through the first transfer conduit; and a second valve configuredto control a flow of the working fluid through the second transferconduit.
 6. The pressure reduction system of claim 2, furthercomprising: a return conduit fluidly coupled with the second transferconduit between the control tank and the alternator; and a pump fluidlycoupled with the return line conduit and configured to transfer theworking fluid out of the pressure reduction system.
 7. The pressurereduction system of claim 1, wherein the control tank is configured tobe maintained at a tank pressure between about 0.5 MPa and about 2 MPa.8. The pressure reduction system of claim 7, wherein the cavity isconfigured to maintain a cavity pressure between about 0.5 MPa and about11 MPa.
 9. A cooling system comprising: an alternator comprising acasing and a rotor positioned, at least in part, in a cavity defined bythe casing; a mass management system comprising a control tankconfigured to be positioned at an elevation above the alternator, thecontrol tank comprising a refrigeration loop configured to cool aworking fluid contained within the control tank; a first transferconduit fluidly coupling the alternator and the mass management systemand configured to transfer the working fluid from the cavity to thecontrol tank; and a second transfer conduit fluidly coupling thealternator and the mass management system and configured to transfer thecooled working fluid from the control tank to the cavity viagravitational force.
 10. The cooling system of claim 9, wherein therefrigeration loop is closed.
 11. The cooling system of claim 9, furthercomprising a heat exchanger fluidly coupled with the first transferconduit and configured to cool the working fluid prior to the workingfluid entering the control tank.
 12. The cooling system of claim 9,wherein the control tank is configured to be maintained at a tankpressure substantially lower than a cavity pressure within the cavity ofthe alternator.
 13. The cooling system of claim 9, further comprising: areturn conduit fluidly coupled with the second transfer conduit betweenthe control tank and the alternator; and a pump fluidly coupled with thereturn line conduit and configured to transfer the working fluid out ofthe pressure reduction system.
 14. The cooling system of claim 9,wherein the working fluid comprises carbon dioxide.
 15. A heat enginesystem, comprising: an expansion device in a working fluid circuit, theexpansion device configured to receive a working fluid at an expansiondevice inlet at a high pressure and to output the working fluid at a lowpressure, and wherein the expansion device converts a pressure drop inthe working fluid to mechanical energy; an alternator fluidly coupled tothe expansion device, the alternator converting the mechanical energy toelectrical energy, the alternator comprising a casing and a rotorpositioned at least in part in a cavity defined within the casing, thecavity further configured to receive a portion of the working fluid fromthe expansion device; a mass management system comprising a control tankconfigured to be maintained at a tank pressure substantially lower thana cavity pressure within the cavity to form a pressure differentialtherebetween; a first transfer conduit configured to transfer theworking fluid from the cavity to the control tank via the pressuredifferential; a pump fluidly coupled to the expansion device andconfigured to receive the working fluid at a low pressure and output theworking fluid at a high pressure; a recuperator fluidly coupled to thepump and configured to heat the working fluid exiting the pump; and awaste heat exchanger fluidly coupled to the recuperator and configuredto further heat the working fluid after exiting the recuperator andbefore entering the expansion device.
 16. The heat engine system ofclaim 15, further comprising a second transfer conduit configured totransfer the working fluid from the control tank to the cavity of thealternator.
 17. The system of claim 16, further comprising: a returnconduit fluidly coupled with the second transfer conduit between thecontrol tank and the alternator; and a transfer pump configured totransfer the working fluid out of the control tank to a location in theworking fluid circuit between the pump and the expansion device.
 18. Thesystem of claim 17, wherein the cavity is configured to receive theportion of the working fluid at a leak rate, and the transfer pump isconfigured to transfer the working fluid to the location in the workingfluid circuit between the pump and the expansion device at a ratesubstantially equal to the leak rate.
 19. The system of claim 17,wherein the mass management system comprises: a third transfer conduitconfigured to transfer the working fluid between the control tank and alocation upstream of the expansion device; and a fourth transfer conduitconfigured to transfer the working fluid between the control tank and alocation upstream of the pump.
 20. The system of claim 17, furthercomprising a second mass management system comprising: a second controltank; a third transfer conduit configured to transfer the working fluidbetween the second control tank and a location upstream of the expansiondevice; and a fourth transfer conduit configured to transfer the workingfluid between the second control tank and a location upstream of thepump.